CA2688069C - Induction heater - Google Patents

Abstract

An induction heater for heating metallic billets 10 with a yoke of E-shaped cross-section, on the middle limb 143 of which a superconducting coil 121 is seated, has a well located between the middle limb 143 and each one of the respective two outer limbs 142 I, 142 r.
A billet 10 can be heated by being rotated in each one of the two wells.

Description

Induction Heater Field of the Invention The invention relates to an induction heater with a direct-current fed superconducting coil arrangement on a yoke, and a method for adjusting the yoke width.

Background of the Invention 1C An induction heater is known from DE 10 2005 061 670.4. For heating a billet of an electrically conducting material the billet is rotated in a well between two limbs of a yoke having a C-shaped cross-section. A direct-current fed high-temperature superconducting coil is seated on the yoke. Designated as being high-temperature superconducting (HTSC) are cuprate superconductors, e.g. YBCO and, more generally, all superconductors (SC) having an SC transition temperature above the boiling point of liquid nitrogen. As a rule, induction heaters are incorporated in a production line.
Therefore the induction heater must provide a heated billet according to timing set by the production fine.

An induction heater with an approximately E-shaped yoke is known from US
5,412,183 A, the three limbs of which are designed as pole pieces and are disposed in a star-shaped configuration at angular displacements of 120 degrees from each other, in order to heat by induction a work-piece in the space between the pole pieces with alternating-current fed coil arrangements seated on the pole pieces.
From FR 904 159 A another induction heater with an E shaped yoke is known, on the middle limb of which a first coil arrangement is seated, and the end limbs of which are directed towards each other. The work-piece to be heated is located between the spaced end faces of the end limbs of the yoke, and is surrounded by another coil arrangement which is alternating-current fed and primarily supplies the power for inductive heating of the work-piece.

Another induction heater having an E shaped yoke is known from EP 266 470 Al, the three limbs of which each support an alternating-current fed coil arrangement in order to heat by induction the work-piece located in the free space between the limbs.

-2-Summary of the Invention The invention is based on the object of providing an induction heater having an increased billet output per unit of time, and a low energy consumption.

This object is achieved by induction heater with at least one direct-current fed superconducting coil on a yoke for heating billets, wherein the yoke has a middle limb located between two outer limbs on a common cross-limb, and wherein a respective well for accommodating one billet to be heated is located between the middle limb and each of the two outer limbs, and by a method for adjusting the width of the wells of an induction heater.

The induction heater according to the present invention has a yoke of at least approximately E-shaped cross-section, consisting of a middle limb between two outer limbs, with the middle limb and the two outer limbs being connected by a cross limb. At least one superconducting coil is seated on one of the mentioned limbs.
Between each of the two outer limbs and the middle limb is a well in which a billet can be heated by being rotated within the well. Because the induction heater has two wells, two billets can be heated at the same time, For example, while a heated billet is being exchanged for a new cold billet, another billet can be heated in the other well. The yield from the induction heater is increased accordingly. The E-shape of the yoke makes it possible to increase significantly the yield of heated billets with only one superconducting coil.
Usually the coil is a part of a coil arrangement which, as a rule, comprises at least also the connecting terminals for the coil.

For example, the coil or the coil arrangement can be seated on the middle limb.
Alternatively also, for example, two coils or coil arrangements can be seated on the cross-limb, with preferably one coil or coil arrangement on each side of the middle limb.
Naturally, one coil can be seated also on each of the outer limbs.

The further developments of the invention described in the following are not restricted to the E-shape of the yoke, and not to the number of wells, in particular.

-3-The two outer limbs and the middle limb of the yoke are connected by a cross-limb.
Preferably the coil arrangement, or the coil, is slide-fitted onto the middle limb until It abuts against the cross-limb. This makes possible a compact yoke with correspondingly shorter magnetic return-flux path, whereby the efficiency of the induction heater is improved.

Preferably the limbs of the yoke consist of solid material. Because the coil is fed by direct current, an expensive structure of a yoke consisting of laminated sheets can be dispensed with, without eddy current losses in the yoke, caused by eddy currents, having to be tolerated. Owing to the absence of laminations which would also provide an electric insulation, the magnetic bulk factor is increased over that of a variant comprising metal sheets. This permits of either an increase of the magnetic field, or a more cost-advantageous structure using simpler materials for the same magnetic field strength.

The coil arrangement preferably comprises an evacuated chamber in which at least one HTSC coil is located. The evacuated chamber makes possible a good heat insulation of the HTSC coil.
The heat insulation is further improved when the HTSC coil is sheathed with a plurality of layers of a metal-coated foil, preferably an aluminum-vapor-coated foil.

The HTSC coil can be supported in the chamber by means of synthetic material bearings.
A heat insulator between the coil arrangement and the open ends of the wells reduces the cooling power needed for the HTSC coil. Particularly suitable are micro-porous heat insulators. A suitable material for the heat insulator is calcium silicate, in particular.

In addition or as an alternative to the heat insulator, an infrared reflector which reflects in the direction towards the billets and is made, for example, of a gold-vapor-coated ceramic can be located in the wells. Heat losses are thereby reduced. Particularly suitable is an infrared reflector of U-shaped cross-section, in the free middle portion of which the billet is rotated.

-4-Preferably an impact protection plate having a high magnetic resistance compared with that of the yoke, e.g. of stainless or special steel (V2A, V4A etc.), is located in front of the coil arrangement in each well. Should a rotating billet become disengaged from Its support, then the impact protection plate prevents the more costly and sensitive superconducting coil arrangement from becoming damaged. Each of the impact protection plates can be seated, for example, in two opposite longitudinal grooves in the associated well.
Preferably the wells are tapered along the direction towards the free ends of the limbs, i.e. the limbs are thickened correspondingly. Thereby the air-gap between the free ends of the limbs, in which the billets are rotated, is shortened. Correspondingly, the magnetic resistance is reduced, and the maximum heating power and efficiency are increased.
The wells may be closed to the environment by a heat insulator. For the billets to be removed from or inserted into the wells, the heat insulator closing the wells is preferably movable.

Additionally or optionally the wells can be dosed to the environment by non-magnetic protective plates. These protective plates prevent a rotating billet which has become disengaged from its clamping device from leaving the well and damaging other machine components, or even injuring persons. Of course, also the protective plates are preferably movable for the wells to be opened.
Preferably the width of the wells can be adjusted. Thereby the wells can be adapted to different billet diameters. This may be effected, for example, by sliding or swiveling at least one lower portion of the outer limbs. The lower portion of the outer limbs also can be segmented in a plane orthogonal to the rotation axis. For effecting an adjustment to the field in a respective well, the segments may be slid or swiveled independently from each other. Alternatively or optionally the widths of the wells can be adjusted with ferromagnetic metal plates interchangeably attached to the limbs of the yoke.

-5-Metal plates of this kind can have a higher relative magnetic permeability than the yoke.
This leads to a concentration of the magnetic flux through the metal plates, and therewith also through the billet being rotated between the metal plates. When especially large billets are to be heated, the metal plates also can have a lower relative permeability than the yoke; the metal plates then act in a scattering manner, and correspondingly the magnetic flux acts more uniformly.

The widths of the wells can increase from the end faces of the yoke towards the middle.
For this, ferromagnetic metal wedges can be interchangeably attached to the limbs of the yoke. This geometry of the wells reduces the stray fields issuing from the wells at the end faces of the yoke, and the magnetic field through the billets is increased correspondingly.

For adjustment of the width of the wells, be it by shifting or swiveling parts of the outer limbs, or also by exchanging interchangeably attached metal plates or wedges, preferably the HTSC coil is first switched off. Subsequently, the width of the wells can be then changed easily. The width of the wells can be changed particularly easily when, after the coil has been switched-off and before the width is changed, the yoke is demagnetized.
For this, for example, a coil arrangement seated on the yoke, in particular the superconducting coil arrangement, can be fed with alternating current. The current strength for feeding with alternating current is lower than the rated current strength for feeding with direct current. Preferably it amounts to about 10 to 20 % of the rated current for feeding with direct current.
Brief Description of the Drawings The drawings illustrate by way of example in a schematically simplified manner an induction heater according to the invention. Shown by Fig. 1 is a partly sectional side view of an induction heater;

Fig. 2 is a cross-section of the magnet unit of the induction heater of Fig.
1;
Fig. 3 is a side view of the magnet unit of the induction heater of Fig. 1;

- 5a -Fig. 4 is a longitudinal section (B/B in Fig. 3) of the induction heater;
Fig. 5 is another magnet unit of an induction heater;

Fig. 6 is a schematic view of another magnet unit seem from below; and Fig. 7 to Fig. 10 are sections through respective induction heaters.
Detailed Description of the Drawings The induction heater In Fig. 1 has a two-part clamping device 2a, 2b which holds a billet 10 in a well of a magnet unit 100. The billet 10 is driven to rotate via a part of the clamping device 2a, a gear unit 3, and a motor 1. The billet 10 can be raised and lowered, as indicated by the corresponding double arrow, by means of the clamping device 2a, 2b. In addition, the clamping devices 2a, 2b can be also adapted to travel horizontally. This is also indicated by double arrows.

6 The billet 10 is located in a well 150 1 of a yoke 140 of E-shaped cross-section, on the middle limb of which yoke a coil arrangement 120 is seated (cf. Fig. 2 to Fig.
4). The yoke is of E-shaped cross-section and has two outer limbs '142 I, 142 r which are joined to a middle limb 143 via a cross-limb 141. Accordingly, there is a shaft 140 I
with open lower end between the outer limb 142 I and the middle limb 143, and another shaft 150 r also with open lower end between the outer limb 142 r and the middle limb 143, The yoke 140 is made of a solid material.

The coil arrangement 120 consists of an evacuated chamber 125 in which an HTSC
coil 121, cooled for example with liquid nitrogen, is located (cooling means and electrical leads are not illustrated). The HTSC coil is located in a housing 122 which is fixed in the chamber 125 by means of a not illustrated synthetic material holder. The =HTSC
coil 121 is located in a housing 122 which is sheathed by a plurality of layers of an Al-vapor-coated polyester foil as a heat insulator. A good heat-insulation is achieved with about 40 to 60 layers of the foil, with about preferably 10 to 20 further layers located at the edges.
An impact protection plate 153 is located below the chamber 125 in each well 150 1, 150 r. The impact protection plates 153 are made of a non-magnetic material, e.g.
stainless or special steel, and are seated in opposite longitudinal grooves in their respective wells 150 I or 150 r. For assembly, the impact protection plates 153 are inserted from one of the end faces of the yoke to slide along the longitudinal grooves 152, and then fastened.
The impact protection plates 153 protect the coil arrangement 120 from being damaged by a rotating billet 10 which has become released from the clamping device 2a, 2b.

Along the downward direction a heat insulator 154, here made of calcium silicate plates, directly adjoins the impact protection plate 153. The heat insulator 154 protects the coil arrangement 120 and the yoke 140 from the heat of the billets 10 in the same way as the adjoining infrared reflector 158 of U-shaped cross-section and gold-vapor-coated ceramic. Furthermore, the losses by heat emission from the billet to the yoke are lessened, The wells 150 1, 150 r are tapered at their lower ends by means of ferromagnetic plates 155 which are interchangeably fastened to the outer limbs 142 I, 142 r, or to the middle limb 143. Thereby the air gap between the limbs 142 I, 142 r and 143 of the yoke 140 and the billets 10 is shortened, and the magnetic resistance of the magnet unit 100 is

7 correspondingly reduced. The plates 155 have a higher magnetic permeability than the yoke 140. Therefore the plates 155 concentrate the magnetic flux through the billets 10.
By comparison with an embodiment in which the wells have a constant reduced width corresponding to the distance between the plates 155, the embodiment shown here has the advantage that the wells 150 I, 150 r are effectively widened along an upward direction, whereby the evacuated chamber 125 is made correspondingly larger and the insulation of the HTSC coil 121 is improved. The interchangeable attachment of the plates 155 makes possible a simple assembly of the magnet unit 100, and also an adaptation of the width of the wells 150 I, 150 r to the diameter of the billets 10 to be heated.

At their lower ends the wells 150 I, 150 r are closed by another heat insulator 156. The heat insulator 156 lies in a channel of three protective plates 157. The protective plates 157 are of a non-magnetic material, for example stainless or special steel, and serve to prevent accidents. Should a billet 10 unexpectedly become disengaged from the clamping device 2a, 2b during the heating, then it cannot leave the corresponding well 150 I, 150 r, which means that it can neither damage other system components, nor injure persons. The heat insulator 156 and the protective plates 157 are adapted to be raised and lowered, as indicated by double arrows. Thereby the wells 150 I, 150 r can be opened in order to insert a billet 10 from below into the corresponding well.

The embodiment in Fig. 5 corresponds substantially to the embodiment in Figs.
1 to 4 (the same or similar parts are indicated by identical reference numerals), however the lower component parts of the two outer limbs 142 I and 142 r are adapted to be displaced in order to conform the width of the wells 150 I, 150 r to billets 10 having different diameters. The displaceable part of the two outer limbs 142 I, 142 r is shown in two positions, with the open position being indicated by a hatching which is counter-directed to the hatching usually employed for the yoke 140.

For adapting the heat insulator 154 and the infrared reflectors 158 to a changed well width, they can be either completely exchanged or adapted to be of telescopically adjustable width (not illustrated).

The magnet unit 100 in Fig. 6 is substantially similar to that of the other induction heaters of the other Figures. Instead of the plates 153 in Fig. 2 and Fig. 5, metal wedges 155b

8 are attached to the outer limbs 142 1 and 142 r and on both sides of the middle limb 143 so as to be exchangeable and displaceable relative to each other. Thereby the width of the wells 150 I and 150 r increases from the end faces towards the middle.
This reduces stray fields that emerge from the end faces and makes it possible to adapt the field to form a field profile. By displacing the metal wedges 155b parallel to the rotation axis it is possible thus to adapt to, for example, different materials or geometries. The efficiency of the magnet unit 100 is correspondingly improved.

The induction heaters 100 in the Figures 7 to 10 are similar to the induction heaters 100 in the Figures 1 to 4. Therefore identical reference numerals are used for the same or similar parts, and more detailed description will be made merely of the differences.

The induction heater 100 in Fig. 7 has a coil arrangement 120 on the right-hand side outer limb 142 r, and a coil arrangement 120 on the left-hand side outer limb 142 I, instead of a coil arrangement on the middle limb 143 as shown in Figs. 1 to 4.

The induction heater 100 in Fig. 8 has merely one coil arrangement which is seated on the left-hand side outer limb 142 I and has been slide fitted onto this until it abuts against the cross-limb 141.
The Fig. 9 shows an induction heater 100 with a coil arrangement 120 that is seated on the cross-limb 141 between the left-hand side outer limb 142 I and the middle limb 143.
To enable mounting of a pre-assembled coil arrangement 120, the left-hand side outer limb 142 I differs from that illustrated by being adapted to be demounted.
Fig. 10 shows an induction heater 100 with one coil arrangement 120 seated on the cross-limb 141 on each of the two sides of the middle limb 143. To enable mounting of a pre-assembled coil arrangement 120, the two outer limbs 142 1, 142 r differ from those illustrated by being adapted to be demounted.

Claims (27)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Induction heater with at least one direct-current fed superconducting coil on a yoke for heating billets, wherein the yoke has a middle limb located between two outer limbs on a cross-limb, and wherein a respective well for accommodating one billet to be heated is located between the middle limb and each of the two outer limbs.

2. Induction heater according to claim 1, wherein the coil is slide-fitted onto the middle limb of the yoke until it abuts against the cross-limb.

3. Induction heater according to claim 1 or 2, wherein at least one limb of the yoke is made of a solid material.

4. Induction heater according to any one of claims 1 to 3, wherein the coil is an HTSC coil in an evacuated chamber of a coil arrangement.

5. Induction heater according to claim 4, wherein the coil is supported in the evacuated chamber by means of synthetic material bearings.

6. Induction heater according to any one of claims 1 to 5, wherein the coil is sheathed with a plurality of layers of a metal-vapor-coated foil.

7. Induction heater according to any one of claims 1 to 6, having a heat insulator between the coil and the open end of the respective well.

8. Induction heater according to claim 7, wherein the heat insulator is micro-porous.

9. Induction heater according to claim 7 or 8, wherein the heat insulator is of calcium silicate.

10. Induction heater according to any one of claims 1 to 9, and having an infrared reflector in each of the respective well.

11. Induction heater according to claim 10, wherein the infrared reflectors are of a U-shaped cross-section.

12. Induction heater according to any one of claims 1 to 11, and having a non-magnetic impact protection plate in each of the respective well.

13. Induction heater according to claim 12, wherein each of the respective well has two oppositely located longitudinal grooves in which one of the impact protection plates is seated.

14. Induction heater according to any one of claims 1 to 13, wherein the respective well is tapered in a direction towards free ends of the limbs.

15. Induction heater according to any one of claims 1 to 14, wherein the respective well is closed-off from its surroundings by a heat insulator.

16. Induction heater according to claim 15, wherein the heat insulator closing-off the respective well is adapted to be moved to open the respective well.

17. Induction heater according to any of claims 1 to 16, wherein the respective well is covered-off from its surroundings by non-magnetic impact protection plates.

18. Induction heater according to claim 17, wherein the protection plates are adapted to be moved to open the respective well.

19. Induction heater according to any one of claims 1 to 18, wherein a width of the respective well is adjustable.

20. Induction heater according to claim 19, wherein the width of the respective well can be adjusted by shifting or swiveling at least parts of the outer limbs.

21. Induction heater according to claim 19 or 20, wherein the width of the respective well is adapted to be adjusted by means of ferromagnetic metal plates interchangeably fastened to the limbs of the yoke.

22. Induction heater according to claim 21, wherein a relative magnetic permeability of the metal plates deviates from that of the yoke.

23. Method for adjusting the width of the respective well of an induction heater as defined in any one of claims 19 to 22, comprising the steps of:
(a) switching-off the coil; and (b) changing the width of the respective well.

24. Method according to claim 23, wherein the yoke is demagnetized after step (a) and before step (b).

25. Method according to claim 24, wherein the yoke is demagnetized by feeding alternating current to a coil arrangement.

26. Method according to claim 25, wherein the coil is fed with alternating current.

27. Induction heater according to claim 10, wherein the infrared reflector is a gold-vapor-coated ceramic.